Usually none. If I do for some sketching before I started my drawing, it somehow always become a much more clean sketch (sometimes I do a full blown clean lineart and coloring) Than it needed and typically makes me doing it for half an hour to two. Which is Why I Didn't do it often.
I've recently started doing really rough ball-point pen doodles as warm ups plus trying to develop a "Visual library" where I draw three versions of the same item like a TV or a cheeseburger. The first two from real photos, and one from my imagination. It's fun.
Listening to heavy metal and sketching a bunch of unusable scribbles before moving on to something more substantial.
The design for the horns on image 3 are brilliant!
Exercise is so important with the lockdown, making sure you get that 30 minutes is even more relevant now more than ever 
I can total emapthise with those feelings, it can be so difficult to draw a line and to know when to stop 
Woah, these are great, i especially like the use of tones in the left image!
Building the memory bank is always important 
and using a ball-point pen means you have to put thought into every stroke because there's no going back. that's living to close to the edge 
OMG i totally got metal vibes from your webcomic!
Yeah, I've recently been applying that "Ballpoint Pen" type mentality to my digital sketching and thumbnailing as well to see if that helps get me more used to and comfortable with digital drawing like I am with traditional drawing mediums.
Warm-up routines are a thing...? I didn't know that...
I don't really have one (I usually don't even have the time...) but I guess sometimes when my brain is overflowing with ideas and I feel kind of paralyzed, I do a quick drawing to get myself into "work mode".
Anything I can sketch and color quickly and have fun with, like this:
I usually do a little warm up illustration or a strip along with various sketches. Sometimes I do some quick gesture drawings or doodle some pictures of interiors I have saved on my computer.
Right now I have a little warm up project I’m trying to do where I redraw 60s/70s horror and giallo film posters in my style because I love a lot of the layouts of that era and I’m using it as an excuse to really study it so that I can make something that feels like one of those posters the cover of my next graphic novel.
I also free-write EVERY morning for no specific amount of time just until I hit at least 1000 words. It usually takes around 20-30 minutes. Sometimes the free-writing session spins off into research sessions that don’t usually have anything to with any current projects.
When my work day is over I’ll try to read for most of the evening and maybe watch a movie and I also consider all of this prep work for the next day of work because usually I’ll be reading something relevant to the work I’m doing or the movie I’ll watch will be in the genre I’m trying to work in.
... messing up the drawing 
I've lost the habit of warming up, 
do it people is good for you!!
When I do it, I just make a lot lines and wiggles and circles, shadings and pressure tests.
Oh, the thing I call the morning routine lol
Get up, feed my furbabies. Then make my coffee, drink while doing my light therapy for few hours. (Can't exactly see during that since I have lights in my eyes). Procrastinate for a bit after then realise is 3pm. (I wake up at like 12pm to do all this) then get to work, girlfriend screams at me to get out of my study at 5-6pm. I cook then do more digital work out in my lounge room. That's my work and "warm-up" routine in a nutshell lol
I clean off my desk, wipe down my cintiq, and make sure there aren't a million coffee mugs everywhere. I'll usually put my phone on the other side of the room.
I haven't been doing any warm up or warm down drawings recently just because I felt like I didn't really need to since I'm quarantined and just..home...all day..., but sometimes just drawing a little something something for social media is a good way to warm up, because you don't really need to get all that hard core to draw a Goku.
Um...what's a "warm up routine"? Lol
Kidding...I rarely do them. I'm at the point where I dont like to waste energy, time, or momentum; a lot of times I'll pretty much "warm up" while working on the actual page or project I'm working on...I'll occasionally do some warm up scribbles before I start working on stuff.
Ah. Warmups. I should do them. But then I forget about them, until i stumble upon mention of them on the web, like now. Do they really help?
I'm wondering the exact same thing...
Warm-up is more like "getting in the right mood to be productive" for me.... and I often end up on Internet procrastinating. The hardest part is getting on the real work (even if it's just sketching, which is rarely before 4pm...), after that I can stick to it for hours but until then I get easily distracted (garden needs some care, cat needs some care, other cat... so many cats!)